British Foreign Secretary William Hague said on Saturday that Russian action was a "potentially grave threat" to Ukraine's sovereignty, independence and territorial integrity.

William Hague said he had spoken to Russian Foreign Minister Sergei Lavrov to urge steps to calm the situation, and had summoned the Russian ambassador to register Britain's concern.

"I am deeply concerned at the escalation of tensions in Ukraine, and the decision of the Russian parliament to authorise military action on Ukrainian soil against the wishes of the Ukrainian Government," William Hague said in a statement.

"We condemn any act of aggression against Ukraine."

He advised all British nationals to leave Crimea immediately by all practical means.